Proximity services (ProSe) are a kind of services provided by a 3GPP system based on user terminals close in distance, which mainly provide functions such as proximity device discovery, device communication, and user equipment (UE)-to-network relay (UE-to-Network Relay). The UE-to-network relay refers to, for example, providing a unicast relay service for a remote UE using a network relay UE. That is, the remote UE communicates with a PDN (Packet Data Network) through the network relay UE. In addition, synchrozation reference and transmission of other communication content may be performed between UEs through device-to-device (D2D) communication.
With regard to the D2D communication, the 3GPP currently focuses on device discovery and broadcast communication. As a basis for device discovery, a D2D synchronization source needs to transmit a D2D synchronization signal (D2DSS) to support D2D synchronization.